1. Field of the Invention
The present invention relates to a constant current driving apparatus which drives each of a lot of loads at a constant current, and a constant current driving semiconductor integrated circuit for use in such a constant current driving apparatus.
2. Description of the Related Art
Traditionally, in a light-emitting diode (abbreviated as "LED" hereinafter) printer head, an LED display panel which are formed by arranging a lot of LED devices, an organic electroluminescence (abbreviated as "EL" hereafter) display panel which utilizes an EL phenomenon of a specific organic compound, and the like, a lot of loads such as light-emitting devices are individually driven at a constant current. Since all these loads need to be driven at a time, they are often driven by using a plurality of constant current driver integrated circuits (occasionally abbreviated as "IC" hereinafter), which are semiconductor ICs provided with a plurality of constant current output terminals. In this case, when variations arise in output current values of the constant current driver ICs, variations arise in light-emitting amounts or the like of the light-emitting devices, print inconsistencies are caused in a printer, and display inconsistencies are caused in a display panel. In order to reduce such inconsistencies, it is required to minimize variations in output currents of the constant current driver ICs among the constant current output terminals.
On the other hand, in ICs produced from the same mask in the same process step and formed on different semiconductor chips, devices such as transistors and resistances formed on the semiconductor chips have different electrical characteristics, respectively, so that the ICs are not highly matched. However, among devices such as transistors and resistances formed on the same semiconductor chip, a relative error of electrical characteristics is small, so that the ICs are highly matched. For this reason, although variations in output currents of the driver ICs are small among constant current output terminals of the same IC, the variations are relatively large among different ICs. Therefore, in the case of using a plurality of driver ICs to drive light-emitting devices of an LED printer head, an LED display panel, an organic EL display panel and the like, it is required to correct variations among the driver ICs.
Variations among driver ICs are corrected by externally mounting a current setting resistance on each driver IC and regulating resistance values of the current setting resistances. Japanese Unexamined Patent Publication JP-A 8-169139 (1996) discloses a prior art of embedding a current setting resistance which is capable of changing a combined resistance value by combination in each driver IC used for driving an LED head, changing a combined resistance value of a combination of resistances so as to correspond to correction data from outside, and changing a constant current value.
In the conventional method of regulating a current setting resistance value and correcting variations among driver ICs used for driving at a constant current, there is a problem that it is hard to automate an assembly operation of assembling display devices and driver ICs in an LED printer head, an LED display panel, an organic EL display panel or the like. Although variations in constant current outputs are decreased by forming driver ICs into a single semiconductor integrated circuit, the size of the semiconductor integrated circuit is large and the area necessary for a semiconductor chip is also large. In addition, general versatility as a driver IC is lost, with the result that the driver IC becomes exclusive to a specific LED head, LED display panel, organic EL display panel or the like. Such a problem is also true in a driver IC which outputs a constant voltage.